Method of manufacturing carbonaceous refractory products

ABSTRACT

CARBONACEOUS REFRACTORY PRODUCTS MANUFACTURED BY A CONVENTIONAL METHOD IS IMMERSED IN A BATH OF MOLTEN TAR PITCH TO IMPREGNATE THE TAR PITCH INTO THE PORES OF THE PRODUCTS, THE IMPREGNATED PRODUCTS ARE HEATED TO ELEVATED TEMPERATURES TO CARBONIZE THE IMPREGNATED TAR PITCH AND THEN THE SURFACE OF THE PRODUCT IS COVERED WITH A LAYER OF GLAZE.

United States Patent 3,682,686 METHOD OF MANUFACTURING CARBON ACEOUSREFRACTORY PRODUCTS Toshio Nakamura and Hidekichi Ozeki, Gifu-ken,Japan,

assignors to Akechi Taikarengo Kabushiki Kaisha, Enaguu, Gifu-ken, JapanN0 Drawing. Filed Oct. 21, 1970, Ser. No. 82,857 Claims priority,application Japan, Aug. 6, 1970, 45/ 68,845 Int. Cl. C04b 41/06, 41/28U.S. Cl. 11746 CC 5 Claims ABSTRACT OF THE DISCLOSURE Carbonaceousrefractory products manufactured by a conventional method is immersed ina bath of molten tar pitch to impregnate the tar pitch into the pores ofthe products, the impregnated products are heated to elevatedtemperatures to carbonize the impregnated tar pitch and then the surfaceof the product is covered with a layer of glaze.

BACKGROUND OF THE INVENTION This invention relates to a method ofmanufacturing carbonaceous refractory product having improved corrosionproof property and sporing proof property.

Cabonaceous refractories are used as bricks for fabricating dischargeports of converters or for metal melting crucibles, because they havehigher resistance to wear at elevated temperatures and because they aremore difiicult to be wetted by molten metals or molten slags whencompared with refractories of the oxide type such as clayishrefractories, refractories containing alumina at high percentage ormagnesia refractories. However, as the porosity of carbonaceousrefractories ordinarily amounts to to they are rapidly damaged by slagor molten metals, so that their useful life is relatively short. It hasbeen proposed to decrease the percentage of graphite for the purpose ofdecreasing porosity, but as this approach decreases the sporing proofcharacteristic of the products, the products tend to crack during theiruse, thus shortening their life.

SUMMARY OF THE INVENTION It is an object of this invention to provide anexcellent carbonaceous refractory products having improved corrosionproof and sporing proof properties.

In accordance with this invention, this object can be attained byproviding a method of manufacturing carbonaceous refractories comprisinga first step of preparing carbonaceous refractories by a conventionalmethod including mixing, moulding, reducing by firing, a second step ofimmersing said refractories in molten tar pitch to impregnate the tarpitch into the pores of the refractories, heating said impregnatedrefractories to carbonize the impregnated tar pitch, and a third step ofapplying a glaze onto the outer surface of the refractories and thenheating the same to fire said glaze.

The first to third steps are performed in the following manner. As abovedescribed, the first step may be any one of well known methods. Thestarting material essentially comprises a carbonaceous substance such asgraphite and silicon carbide, but if desired, suitable quan- 3,682,686Patented Aug. 8, 1972 tities of oxide refractories such as chamotte,pagodite, silimanite or synthetic mulite; a binder such as clay orpitch, etc.; and a sintering agent such as borax, boric acid, frit,ferrosilicon and the like may be incorporated. These raw materials arepulverized to have a suitable particle size and are then mixed together.After incorporating a suitable quantity of water and tar, the mixture iskneaded, shaped, and reduced by firing. Where water is incorporatedafter kneading the shaped body is dried and then reduced by firing.

In the second step, the tired body obtained by the first step isimmersed in a bath of hot molten tar pitch to impregnate the tar pitchinto the pores of the fired product to fill most of the pores.Preheating of the fired product and evacuation of the impregnationchamber enhance the impregnation. The impregnated body is taken out ofthe chamber and is then heated to evaporate off about half of theimpregnated tar pitch and to carbonize the remaining half, thusdepositing it in the pores as amorphous carbon. This decreases theporosity and increases the mechanical strength of the product and henceincreases the corrosion proof property as well as the wear resistantproperty. On the other hand, the precipitated carbon remains in thepores, in spite of the decrease in the porosity, sporing proof propertywould not be affected.

It is well known in the art to impregnate magnesia bricks for liningsteel mixing furnaces and stopper bricks of clay with tar and then heatthe bricks to deposit carbon in their pores for the purpose ofincreasing their useful life.

However, the carbon deposited in this manner is amorphous and especiallyinflammable so that this method is effective only when the heatreceiving surface of brick is heated in the absence of oxygen as in thecase when the refractory prepared in this manner is used in contact withhigh temperature molten metal. Accordingly, when the refractory isheated from outside by flame or electric heat while the refractory ismaintained in contact with oxygen as in a metal melting crucible, thedeposited carbon will be lost by oxidation, thus losing the effect oftar impregnation.

To eliminate this problem, according to this invention, a glaze isapplied onto the surface of the refractory "prepared by the second stepand the glazed refractory is fired. This third step provides an airimpermeable layer on the surface of the product so that the carbondeposited by heating the impregnated tar pitch is not oxidized by airduring operation at elevated temperatures thus preserving the lowporosity and low air permeable properties of the product over a longperiod of use.

In addition to the reasons described above, the glaze applied by thethird step greatly improves the wear resistant and corrosion resistantproperties of the product on the following ground. More particularly,the carbon deposited in the pores, especially in those near the heatreceiving surface of the product, is converted into hard andnon-inflammable state by the high temperature heating utilized to firethe glaze and by the heating in the absence of oxygen encountered duringthe use of the product. Further, it is considered that a portion of thedeposited carbon is diffused into the glassy composition of the productto render it more non-inflammable and resistant against corrosion.

The glaze can be applied in the same manner as the conventionalceramics. It is advantageous to use a glaze of the borosilicate typewhich does not lose its viscosity at elevated temperatures, for example,a fine powder of 4 with tar pitch, heated to carbonize, applied withglaze and then fired to produce a novel brick product. Following tableshows various characteristics of the brick before and afterimpregnation.

borosilicate glass. Although dependent upon the composition of theglaze, the firing temperature normally ranges from about 1000 to 1100 C.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples aregiven by way of illustration and not limitation.

EXAMPLE 1 35% of natural graphite, 40% of silicon carbide, offerrosilicon and 10% of borosilicate frit (all in weight ratio) weremixed together and 40%, based on the weight of the mixture, of a mixtureof coal tar and pitch was incorporated to the first mixture, and theresulted mixture was heated, kneaded, shaped and fired to reduce at atemperature of 1320 C. to obtain a graphite crucible, 52 cm. high, 35.5cm. in diameter at the top opening, 22 cm. in bottom diameter and 25 cm.in wall thickness. The crucible was then put in a vessel, the vessel wasevacuated and then molten tar pitch was poured into the evacuated vesselto impregnate the soft pitch into the pores of the crucible. Theimpregnated crucible was taken out of the vessel, and heated at atemperature of 800 C. in a reducing atmosphere to remove the volatilecomponent and to carbonize the impregnated pitch. After cooling, amixture of a powder of borosilicate frit and water was sprayed onto thesurface of the crucible so as to coat and impregnate the crucible withthe mixture. Then the crucible was heated at a temperature of 1050 C. tofire the frit to obtain the finished crucible.

Before impregnation the crucible had a weight of 34 kg., a volume of18,400 cm. the total volume of the pores of 4,500 cm.. The quantity oftar pitch impregnated was 5.4 kg., and the quantity of carbon remainedafter the heat treatment was 2.8 kg. Following table shows variouscharacteristics of the ceramic before and after im- The coefiicient ofhot linear expansion was obtained by heating the crucible from normaltemperature to 800 C.

These two types of bricks were used to fabricate discharge ports of 100ton pure oxygen converters and it was found that the bricks notimpregnated were durable for only 400 refining operations whereas thoseimpregnated could be used for 600 refining operations.

Thus, it will be clear that the invention provides novel carbonaceousrefractory products having lower porosity, higher bulk specific gravity,bending strength and compression strength than the prior carbonaceousrefractory products by the deposition of carbon in the pores of theproducts. Further, as the deposited carbon is protected against hightemperature oxidation by the non-permeable layer of the glaze applied onthe surface of the products, it is possible to efiiciently prevent theimpregnation of the fiux, slag and molten metal during operation.Moreover, as the mechanical strength is improved and as the layer ofgraphite, one of the principal compositions, is prevented frome peelingoff, the useful life of the products is greatly prolonged.

What is claimed is:

1. A method of manufacturing a pore-filled glazed carbonaceousrefractory product which comprises the steps of kneading, shaping andfiring a composition essentially consisting of carbonaceous material,and pulverized silicon-containing sintering agents, thus obtaining afired, shaped, porous carbonaceous refractory body, immersing saidfired, shaped, porous carbonaceous refractory body in molten tar pitchso as to substantially impregnate the pores of said body with the moltentar pitch heating the thus impregnated shaped, carbonaceous refractorybody to a temperature suificicntly high to carbonize said tar pitch,thereby obtaining said carbonaceous refractory body with amorphouscarbon deposited in the pores thereof, applying a layer of pregnation.glaze on the surface of the thus obtained carbonaceous Compres- Poros-Percentage Apparent Bulk Bending sion it of water specific specificstrength, strength, percent absorption, gravity gravity kgJcm. kgJcm.

Alter impregnation. 13. 4 8. 2 2.30 1. 96 148 380 Before irnpregnatiom24. 5 13. 3 2. 45 1. 85 92 245 These two types of crucibles were used tomelt Kelmet (an alloy containing 70% of steel and 30% of lead). Thecrucible not impregnated showed a life of 80 hours whereas afterimpregnation one crucible had a life of 142 hours and the other 138hours.

When used to melt borax the crucible not impregnated showed a usefullife of 130 hours where as after impregnation, the novel crucible showeda useful life of 244 hours.

EXAMPLE 2 refractory body having amorphous carbon deposited in the poresthereof, and firing said layer of glaze, thereby forming an airimpermeable layer which retains said deposited carbon in the poresthereof.

2. The method according to claim 1 wherein said composition consists of30 to 35%, by weight, of natural graphite, 40 to 50%, by weight, ofsilicon carbide, 10 to 15%, by weight, of ferrosilicon, 5 to 10%, byweight of borosilicate and 40%, based on the total weight of the abovedescribed ingredients, of a mixture of coal tar and pitch.

3. The method according to claim 1 wherein said glaze comprises a powderof borosilicate frit.

4. The method according to claim 1 wherein said body impregnated withsaid molten tar pitch is heated at a temperature of 800 C. in a reducingatmosphere to carbonize at least a portion of the impregnated tar pitch.

5 6 5. The method according to claim 1 wherein said 2,733,592 2/1956Burchenal et a1. 117--70 X glazed body is fired at a temperature of 1000to 1100 C. 2,488,251 11/ 1949 Wood 117-92 X 3,503,763 3/1970 Mills117-125 X References Cited I 5 WILLIAM D. MARTIN, Primary ExaminerUNITED STATES PATENTS M. R. LU'SIGNAN, Assistant Examiner 2,911,31911/1959 Peter 11746 1,402,412 11/1922 Friedrich 117-123 X US. Cl. X.R.1,628,910 5/1927 Prouty et a1 117-125 X 11770 A, 92

